The present invention relates to a sensor element for a gas sensor, especially for determining the concentration of gas components in gas mixtures.
Platelet-shaped ceramic sensor elements, which are produced by sintering at least one ceramic solid electrolyte foil provided with functional layers, are used, for instance, as lambda probes for determining the oxygen content in the exhaust gases of internal combustion engines. The solid electrolyte foils of these sensor elements are oxygen ion-conducting, and are printed in the unsintered condition with functional layers (electrodes, circuit-board conductors, heating conductors and the like), laminated together to a green body and then sintered at a temperature of 1400xc2x0 C., for example.
Instead of a laminate of ceramic solid electrolyte foils, solid electrolyte layers can also be used, which are printed one over the other in individual printing steps, together with the functional layers onto a supporting substrate,
The named sensor elements are exposed to the hot exhaust gas stream of the internal combustion engine, which has various temperatures. Because of the temperature changes in the exhaust gas stream, which occur suddenly and with various intensities, the sensor elements experience thermal shock, resulting in mechanical tension in the surface area, especially at the edges of the sensor elements.
To raise the shock resistance of the sensor elements, it is known from U.S. Pat. No. 5,144,249 to break the longitudinal edges, i.e., to provide each with a chamfer, the chamfer being applied at an angle of about 45 degrees. The application of the chamfers is performed with a grinding procedure on the already sintered sensor element.
Investigations have revealed that, in spite of the chamfer, during heating of the sensor element, stress cracks still occur. A condition of maximum tensile stress in the chamfer appears as soon as a few seconds after switching on the heating element integrated into the sensor element. During this, the stress cracks start especially from the large surface side edge of the chamfer, and appear particularly when abrupt cooling of the surface of the sensor element occurs because water from condensation impinges on it.
The present invention is based on the object of further improving the shock resistance of planar, ceramic sensor elements.
The sensor element according to the present invention, has the advantage that the thermal shock resistance of the planar sensor element is improved. It could be determined that, especially at the critical edge of the chamfer lying nearest to the heat source, stress cracks no longer appear.
In consideration of the varying heat conducting capabilities of the heating conductor made of platinum-cermet, of the insulating layers made of Al2O3 for the heating conductor and of the solid electrolyte foil made of stabilized ZrO2, an edge geometry was found for the sensor element, which realizes a substantially uniform warming of the large surface side edge lying closest to the heat source, and thereby a symmetrical tension condition is generated at this edge of the chamfer. The edge geometry takes into account the various heat conducting capabilities of platinum, or rather Pt-cermet of ca. 70 W/Km, of Al2O3 insulating layers of ca. 10 W/Km and of ZrO2 foil of ca. 5 W/Km. It was also found that lesser tension conditions occur at a blunt chamfer. In this connection, the edge of the chamfer lying closest to the heat source was selected as more blunt, meaning that this edge forms a larger angle.
Advantageous further developments of the sensor element according to the present invention come to light. The large contact surface of the heating element relative to the large surface of the heater foil involves a large heating current. Therefore it becomes expedient to select the width of the sealing frame larger than the thickness of the heater foil. Thus, additionally, the distance of the surface of the chamfer from the heating conductor is increased even with a flat chamfer in relation to the large area of the heater foil, a distance as large as possible from the heating conductor, for insulating the heating conductor, is realized. A heater foil as thick as possible also guarantees that the critical edge on the large area side of the chamfer lies farther away from the heating element. Because of the lesser deflection of the sensor element resulting from this, lower tensile stresses develop.
It further turned out expedient to form a multiple chamfer, particularly a double chamfer at the edge of the sensor element, with the flat chamfer at the large area of the sensor element running out to a steeper chamfer on the narrow side. It also turned out that edgy transitions between the surfaces should be avoided by rounding off the edges. In order to avoid cracks at the edges of the front face, it is further advantageous to furnish the edges at the front face, too, with appropriate chamfers, and here too, it is especially advantageous to form the sealing frame at the front face wider than the thickness of the heater foil.